The Role of Nitrogen Dioxide in the Production of Sulfate During Chinese Haze-Aerosol Episodes Lijie Li, Michael R Hoffmann, and Agustin J
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Subscriber access provided by Caltech Library Article The Role of Nitrogen Dioxide in the Production of Sulfate during Chinese Haze-Aerosol Episodes Lijie Li, Michael R Hoffmann, and Agustin J. Colussi Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05222 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. 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However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 23 Environmental Science & Technology 1 The Role of Nitrogen Dioxide in the Production of Sulfate during Chinese Haze-Aerosol 2 Episodes 3 4 Lijie Li, Michael R. Hoffmann and Agustín J. Colussi* 5 Department of Environmental Science & Engineering, California Institute of Technology, 6 Pasadena, CA 91125, United States 7 8 9 Haze events in China megacities involve the rapid oxidation of SO2 to sulfate aerosol. Given the 10 weak photochemistry taking place in these optically thick hazes, it has been hypothesized that 11 SO2 is mostly oxidized by NO2 emissions in the bulk of pH > 5.5 aerosols. Since NO2(g) 12 dissolution in water is very slow and aerosols are more acidic, we decided to test such + 13 hypothesis. Herein, we report that > 95% NO2(g) disproportionates: 2 NO2(g) + H2O(l) = H + - 14 NO3 (aq) + HONO (R1), upon hitting the surface of NaHSO3 aqueous microjets exposed to - 15 NO2(g) for < 50 µs, thereby giving rise to strong NO3 (m/z = 62) signals detected by online - - 16 electrospray mass spectrometry, rather than oxidizing HSO3 (m/z = 81) to HSO4 (m/z = 97) in 17 the relevant pH 3-6 range. Since NO2(g) will be consumed via R1 on the surface of typical - 18 aerosols, the oxidation of S(IV) may in fact be driven by the HONO/NO2 generated therein. 19 S(IV) heterogeneous oxidation rates are expected to primarily depend on the surface density and 20 liquid water content of the aerosol, which are enhanced by fine aerosol and high humidity. - 21 Whether aerosol acidity affects the oxidation of S(IV) by HONO/NO2 remains to be elucidated. 22 23 es-2017-05222n-R2 1 ACS Paragon Plus Environment Environmental Science & Technology Page 2 of 23 24 25 26 TOC 27 28 29 2 ACS Paragon Plus Environment Page 3 of 23 Environmental Science & Technology 30 1. Introduction 31 Chinese megacities often experience haze events (HE)1-7 that severely impair visibility and 32 induce acute health effects.5-6, 8-11 Hazes mainly consist of sulfate aerosols produced in the 33 atmospheric processing of SO2 and NO2 emissions under particularly adverse meteorological 34 conditions.12-14 Current models of chemistry in HE significantly underestimate sulfate formation 15-17 35 revealing that the mechanism of SO2 oxidation is not well understood. The drastic 18-21 36 attenuation of actinic radiation in hazes, suggests that SO2 is oxidized by NO2(g) in 37 heterogeneous processes on the aerosol itself, rather than in the gas-phase by photogenerated 38 oxidants. The details of such process are uncertain. 39 Known SO2 atmospheric oxidation pathways include gas-phase reactions with OH-radicals 22 40 and stabilized Criegee intermediates, and aqueous phase reactions with O3, H2O2, organic 41 peroxides, NOx, as well as autoxidation catalyzed by transition metal ions. The low 18, 23-30 42 concentrations of photogenerated oxidants implies that the chemistries of SO2 and NO2 in - 43 HE are intertwined. These observations have led to hypothesizing that HSO3 (aq) is rapidly 44 oxidized by the NO2(g) dissolved in aqueous aerosol phases assumed to be at pH > 5.5, reaction 45 R0: - + - - 46 2 NO2(aq) + HSO3 (aq) + H2O(l) = 2 H (aq) + HSO4 (aq) + 2 NO2 (aq) (R0) 47 on the basis of R0 rates measured in bulk water.31, 32, 33 Good agreement between field 48 observations and model results on sulfate aerosol formation in HE could be obtained by 49 assuming that R0 proceeds on the surface pH > 5.5 aqueous aerosols at the rates previously 50 reported at pH ∼ 6 in bulk water.31-34 However, the assumptions that aerosols are at pH > 5.5, and 51 NO2(g) dissolves in large surface-to-volume aerosol droplets as NO2(aq) as it does in bulk water 52 may not apply.35-45 Most recent studies suggest that HE aerosols are in fact in the pH 3-5 3 ACS Paragon Plus Environment Environmental Science & Technology Page 4 of 23 46-52 53 range. Previous laboratory experiments have shown that collisions of ppm NO2(g) (i.e., in the 54 absence of N2O4) with the surface of aqueous electrolyte solutions (but not on the surface of pure - 55 water) yield NO3 (aq) via a first-order in [NO2] hydrolytic disproportionation catalyzed by 42, 45 56 anions, reaction R1. + - 57 2 NO2(g) + H2O(l) = H + NO3 (aq) + HONO (R1) -1 58 The facts that NO2(g) is weakly soluble in pure water (Henry’s law constant H ∼ 0.01 M atm ) 59 and its uptake coefficient on pure water very small: γ ∼ 1 × 10-7,53 its dissolution) in water is 60 unfavorable both by kinetic and thermodynamic reasons. The aqueous phase of most 61 atmospheric aerosols, however, is not pure water. In 2009, we found that anions greatly enhance - 62 NO2(g) uptake on water. The rate determining step involves the capture of NO2(g) by anions X - 42, 45 - 63 as X-NO2 at air-aqueous interfaces, followed by the reaction of X-NO2 with a second 3-4 64 NO2(g). This phenomenon accounts for the outstanding discrepancy (by a ∼ 10 factor) between 41, 53 65 the NO2(g) uptake coefficients measured in neat water vs those determined on NaCl-seeded 66 droplets in a cloud chamber.35, 39, 54 Since HE aerosols naturally contain organic and inorganic 67 anions, the expectation was that the fate of NO2(g) would be determined by R1 at the relevant 68 aerosol air-aqueous interfaces. 69 Given the societal and economic impact of HE, we deemed important to elucidate the actual 70 role of NO2(g) in the production of sulfate aerosol under relevant conditions. Herein we report 71 experiments in which aqueous 1 mM NaHSO3 microjets (containing 3 mM of added EDTA to - 72 inhibit the potential autoxidation of HSO3 catalyzed by pervasive transition metal ions) ejected 73 from a stainless steel syringe are exposed to 5 ppm NO2(g) for ≤ 50 µs in 1 atm of N2(g) at 298 74 K. Reactant and product ions formed on the outermost water layers of the liquid microjets are 75 detected within 1 ms by online electrospray ionization mass spectrometry (o-ESI-MS). This 4 ACS Paragon Plus Environment Page 5 of 23 Environmental Science & Technology 76 technique has been used in our laboratory to investigate a suite of gas-liquid reactions at the air- 55-61 77 water interface. The analysis of our experimental results focuses on the fate of NO2(g), and - 78 the competition between R0 vs R1 during NO2(g) collisions with the surface of aqueous HSO3 79 solutions under conditions relevant to HE. To our knowledge, this is the first study to provide 80 direct experimental evidence on the extent of S(IV) oxidation by NO2(g) on the surface of 81 aqueous electrolyte solutions over a wide pH range. 82 2. Methods - 83 Reactive interactions of NO2(g) with HSO3 (aq) are investigated on the fresh surface of - 84 continuously flowing HSO3 (aq) microjets that are crossed by NO2(g)/N2(g) beams in the spray 85 chamber of an electrospray ionization (ESI) mass spectrometer maintained at 1 atm of N2(g), and 86 298 K (Agilent 1100 Series G2445A Ion Trap LC-MS-MSD) (Figure 1). This experimental 44-45, 55, 57-58, 60-64 87 setup has been described in more detail in previous reports from our laboratory. 88 Aqueous 1 mM NaSO3H solutions (containing 3 mM EDTA to chelate pervasive traces of - −1 89 transition metal ions known to catalyze HSO3 autoxidation) are pumped at 50 µL min through 90 an electrically grounded stainless steel needle injector (100 µm bore).